1. Chromosomal mapping of human immunoglobulin genes and several human onc gene analogues has been carried out using somatic cell hybrids. The precise localization of certain proto-oncogenes has been established by direct in situ hybridization to human chromosomes in both normal and neoplastic cells. 2. Continuous murine B-cell lines have been established from Rauscher leukemia virus-infected cells. These cell lines fall into five categories, each representing a different developmental stage.